This dude gets a parenting win. He taught his kid murder martial arts, but the kid is self aware and doesn't really want to even risk hurting other by participating in recreational versions of martial arts since he could slip up. It's not like he could've known the kid would actually end up in a situation where he needs to fight for his own life, but he prepared him for it just in case. Even discounting the far fetched possibility of getting isekai'd who knows what'll happen any given day, right? War could come to your country unexpectedly, or on the smaller scale just some stupid dispute between individuals could occur.